Jimi Hendrix is probably the one who has meant the most for spreading appreciation of screaming guitar amplifiers, which is nowadays an effect used by all guitarists, from amateurs to professionals. The feedback effect is physically achieved when the sound from the speakers stimulates the guitar string through the room's acoustic response, which in turn affects the speaker and so forth. FIG. 1 illustrates this feedback. Consequently, a rather high volume and short distance between guitar and speaker is needed for that to take place. This so called feedback can only be stopped by reducing the amplification to the speaker, or increasing the distance between speaker and guitar.
A practical problem for guitarists is that it is complicated to rehearse feedback effects, since high volume is necessary. For this reason, headphones, for example, can not be used. The room acoustics also affect the effect, so that, in principle, the guitarist must practice the feedback effects on the stage or in the studio where the effect is to be presented. It would therefore be of great practical interest to enable simulation of such effects and to be able to experiment in any environment using a low volume.
Acoustic feedback is an example of a feedback system with positive feedback, which makes the closed loop system unstable. The theory of feedback systems is described in all textbooks in the field of control theory, for example the textbook T. Glad and L. Ljung, Reglerteknik, grundldggande teori (Studentlitteratur 1989). There are currently various different control loops in use, ranging from track control and revolution control in CD players, steering servos and ABS systems in cars, to the hundreds of loops used by all process industries to control flows, temperatures, concentrations, etc. In all cases described in the literature, feedback is used to stabilize the system to be controlled. The present application to destabilize the acoustic system may therefore be seen as rather unique, for which no complete theory exists.
In order to simulate the whole physical chain in FIG. 1, a model of the amplifier, speaker, room acoustics and string dynamics is needed. How different parts in this chain can be modeled is described in textbooks concerned with modeling and system identification, for example L. Ljung and T. Glad, Modeling of dynamic systems, L. Ljung, System identification, Theory for the user (Prentice Hall, Englewood Cliffs, N.J., second edition, 1999), T. Söderström and P. Stoica, System identification (Prentice Hall, New York, 1989).
If this is done according to the text books, one does indeed get an unstable system, but one which does not sound anything like the true feedback effect. Common linear feedback system's theory, T. Glad and L. Ljung, Reglerteknik, grundldggande teori (Studentlitteratur 1989), states that the signal amplitude very quickly approaches infinity, which lacks physical meaning. Accordingly, there is a need for nonlinear models and more advanced linear theory such as T. Glad and L. Ljung, Reglerteori, flervariabla och olinjdra metoder (Studentlitteratur 1997) or D. Atherton Nonlinear Control Engineering. 
Earlier patents within this field all modify the guitar in one way or the other:                U.S. Pat. No. 6,681,661 dynamically modifies the opening to the string instrument's cavity.        U.S. Pat. No. 5,449,858 includes a coil device which is attached to the hand of the player, affecting the sound and feedback.        U.S. Pat. No. 5,233,123, U.S. Pat. No. 4,941,388, U.S. Pat. No. 4,852,44, DE4101690 all give examples of so called sustainers, which prolong the tones with electromagnetic transmitters (so called transducers) that directly affect the strings.        U.S. Pat. No. 4,697,491 gives an example of an electrically feedbacked guitar equipped with an electromagnetic transmitter on the neck.        